Rotax 912 uls fuel pump gauge?

[SilverFox5]

Hello,
We are building a K5 and have a slightly used 912uls (410 hrs) that came with a panel mounted fuel pressure gauge. We have decided we will not install an electric back up pump. My question is, if we install the panel fuel pressure gauge will this show/tell us if the mechanical fuel pump is working? Anyone running a pressure gauge w/o a backup electric pump? Is there an obvious difference in fuel pressure when the engine is running or not running and the gauge is just reading the pressure developed by the wing tanks?
Thanks in advance for any feedback.

Joe Leeman
Freedom, WI

[Dave S]

Joe,

Your install as planned will show if the mechanical pump is working.

The head pressure from the tanks with the engine not running is not very much and will not interfere with live pressure readings from the mechanical fuel pump.

A little thing to be aware of - if you do not have the pressure relief valve like the newer systems have, after shutdown the fuel pressure will still register on your gauge since the mechanical fuel pump won't allow the pressure to bleed back. On a hot day the pressure may rise a bit after shutdown as the fuel between the carbs and fuel pump expands with the hot soak. Not a big deal just to know it's normal if you don't have the bleed valve.

I don't have a fuel pressure gauge installed; however I do check the pressure on annual condition inspection to be sure it is within spec. Since we do have the electric pump - the pressure will rise a bit but does stay within spec if the engine is running and the electric is on at the same time. Rotax has an upper limit for fuel pressure (which I don't recall offhand). I think Rotax had some mechanical pumps they built at one time which exceeded the upper pressure limit and caused troubles.

You should be good to go:)

[SilverFox5]

Hi Dave,
Thanks for the good info.
Also just in case I decide to use a backup electric fuel pump plumbed in series to the engine would I need a return line to the header tank? I ask because the engine I have also came with a facet pump.
Thanks again.
Joe Leeman

[Dave S]

Joe,

I believe the people who have installed the Facet pump plumb it in series - that was what I did placing the pump under the center console.

I am not aware of anyone using a return line because of installing the electric pump. A return line is used on the newer Rotax engines in conjunction with a calibrated orifice to relieve pressure between the mechanical pump and the carbs to help prevent flooding in hot soak conditions after shutdown.

The basic Kitfox fuel system is designed without an electric backup pump and a case can be made for either leaving it out per the original Kitfox plan or installing one. The electric pump can't solve every possible failure mode of the mechanical pump. It's a builder's discretion item. I do not normally run with the pump on; but, use it on startup to help with a quick start; and as a standby backup for the mechanical pump.

[jiott]

I run a 912uls without an electric backup pump and I do have a fuel pressure gauge. As Dave says, the head pressure from the wing tanks without engine running is quite small, on the order of ~1 psi, so yes the gauge shows mechanical pump pressure when the engine is running. I also have the strongly recommended return line (thru a small orifice) back to the header tank. This return line's main purpose is not to prevent flooding or trapped pressure, but to reduce possibility for vapor lock by recirculating some cool fuel from the header tank especially after a shut down and hot soak.